Method of producing tungsten rhenium alloys by chemical vapor deposition

ABSTRACT

In a chemical vapor deposition process wherein a plurality of refractory metals is deposited on the surface of a substrate, the usual columnar grain structure of the deposited metals inherent in chemical vapor deposited coatings is rearranged to form an alloy of such metals having a strong, equiaxed grain structure, by depositing the metals in extremely thin layers wherein all adjacent layers are predominately of different metals, and then heattreating the resultant coating. As a result, diffusion of the metals through substantially all adjacent layers, and rearrangement of the grain structure occurs.

United States Patent [151 3,637,374 Holzi et al. 1 Jan. 25, 1972 METHOD OF PRODUCING TUNGSTEN I Reierences Cited RIIENIUM ALLOYS BY CHEMICAL UNITED STATES PATENTS VAPOR DEPOSITION 3,113,376 12/1963 Pflumm et al.. ..75/l35 [72] Inventors: Robert A. Holzi, La Canada; Frederick A. 3'236'699 Pugh al "75/176 Glaski, Chatswonh; James Humphrey, 3,399,98l 9/l968 Maykuth et al. ..75/176 X North Hollywood, all of Calif. Primary Examiner-Charles N. Lovell [73] Assignee: Fansteel Metallurgical Corporation, North Attorney-Stanley Bialos Chicago, Ill. 22 Filed: May 27, 1968 [571 ABSTRACT [21] Appl 732,308 ln a chemical vapor deposition process wherein a plurality of refractory metals is deposited on the surface of a substrate, the usual columnar grain structure of the deposited metals inherent in chemical vapor deposited coatings is rearranged to [52] U.S. Cl ..75/174, 75/135, 11463403426, form an alloy of such metals having a Strong equiaxed grain structure, by depositing the metals in extremely thin layers {2;} 335.5;as;':JJJ:31::JJ:3:;jjjjiygiifilffiiff,i??? ayes of differ" l48/l27; ll7/62, I07, 107.2; l64/l, 46, 76

metals, and then heattreating the resultant coating. As a result, diffusion of the metals through substantially all adjacent layers, and rearrangement of the grain structure occurs.

6 Claims, 6 Drawing Figures METHOD OF PRODUCING TUNGS'IEN Rll-IENIUM ALLOYS BY CHEMICAL VAlOR DEPOSITION This invention relates to formation of alloys by chemical vapor deposition (hereinafter designated as CVD), and more particularly to depositing the metals in extremely thin layers in which adjacent layers are predominately of different metals, and then heattreating the resultant coating or plating to cause diffusion of the metals and rearrangement of the grain structure to a strong equiaxed alloy of such metal.

BACKGROUND OF THE INVENTION It is known in the prior art to form alloys of metals, for example, refractory metals such & rhenium and tungsten, by the so-called powder metallurgy method. in such method an extremely fine powder of the different metals, such as rhenium and tungsten, wherein the powders are so fine that they can pass a screen in the order of one micron mesh, are compressed together and sintered at a high temperature. At the high temperature, rearrangement of the grain structure is obtained to form a strong equiaxed grain in the nature of a truly wrought metal. In this connection, an Equiaxed Grain Structure is one in which the grains have approximately the same dimensions in all directions, as is defined in the American Society of Metals" handbook, 8th Edition, 1961, on page 15, published at Menlo Park, Ohio.

Although the compression and subsequent sintering method, will produce a strong equiaxed grain structure, it is time consuming and requires relatively costly apparatus. Moreover, by such method the alloy cannot be molded readily in the form of objects having curved or other wise irregularly shaped surfaces, such as refractory metal crucibles; nor is it applicable to the manufacture of tubes having a very small inside diameter of about three-sixteenths inch or less. It is only suitable for production of sheets, or relatively large inside diameter tubing of constant diameter.

SUMMARY AND OBJECTS Summarizing the invention hereof, alloys of metals are produced by chemical vapor deposition by depositing the metals by an suitable CVD technique on the hot surface of an object such as a mold or substrate in layers wherein adjacent layers are predominately of different metals. The respective layers deposited are so thin that upon subsequent heat treatment the metals in such respective layers uniformly diffuse among each other with consequent rearrangement of the grain structure, thus forming an equiaxed grain alloy.

If the layers are too thick, then diffusion and consequent grain structure rearrangement will not be effected. The layer thickness should be below about microns (about 0.001 inch equal 1 mil), and the thinner the better because the thinner the layers the more effective is the diffusion of the different metals upon subsequent heat treatment. It is immaterial how many layers are deposited, as the number of layers is merely dependent on the overall coating thickness desired.

The usual temperatures commonly employed in CVD are applicable herein, as the invention is not dependent on particular vapor deposition temperatures as long as temperatures employed are sufficient to cause plating of the metal layers on the hot surface of the substrate or mold. It is only necessary that the deposition on the hot surface mold be effected in discrete extremely thin layers. In this connection, conventional vapor deposition techniques comprise depositing the metal onto the hot surface by reaction of a metal in the gaseous halogen state, such as the hexafluoride, and a reducing agent, such as hydrogen, in a reaction chamber under a vacuum which effects flow of the gaseous reactants into the chamber.

The plurality of metals to be alloyed, such as rhenium and tungsten, can be simultaneously codeposlted on the hot surface, in which case the individual layers will be predominately of different metals because one of the metals (rhenium) plates out first on the hot surface. Instead of codepositing the metals to be alloyed by the subsequent heat treatment, the separate layers can be of pure metals, in other words, alternate layers of rhenium and tungsten in the case of an alloy of these metals.

Under such circumstance, the respective individual layers will be entirely of one metal, instead of a mixture of metals in which the respective layers are constituted primarily of one metal. Hence, the expression that the adjacent layers are predominately of different metals is employed herein in a generic sense to include separate layers of pure metal such as adjacent layers of pure rhenium and tungsten, or layers wherein in each layer one of the metals constitutes a major amount. In either event, the diffusion will occur upon subsequent heat treatment as long as the layers are sufficiently thin, as explained above.

As will be pointed out hereinafter in greater detail, it is immaterial how the layered structure is formed as long as the layers are very thin. Also, the heat treatment time and temperature are relatively immaterial as long as sufficient heat energy is imparted to the layered structure to cause the grain transformation as in conventional sintered metallurgy technique.

From the preceding, it is seen that the invention has as its objects among others, the provision of an improved method for forming strong equiaxed grain structures in an alloy of metals, desirably refractory metals, such as rhenium and tungsten, by CVD and to an improved product resulting from such method.

Other objects of the invention will become apparent from the following more detailed description and accompanying drawings in which:

FIG. l is a photomicrograph of a typical codeposited 20 percent rheniumpercent tungsten layered structure in asdeposited" condition l0O-power magnification);

FIG. 2 is a photomicrograph of the layered structure of FIG. I after heat treatment at 2,000 C. for 10 minutes (also power magnification);

FIG. 3 is a photomicrograph of a 23 percent rhenium-77 percent tungsten layered structure in the as-deposited" condition, in which adjacent layers are separately deposited as pure tungsten and pure rhenium so that the alternate layers are of the same metal (400-power magnification);

FIG. 4 is a photomicrograph of the layered structure of FIG. 3 after heat treatment at 2,000 C. for 10 minutes (also 400- power magnification);

FIG. 5 is a schematic sectional elevation of a reverse flow CVD system of the type illustrated in assignees now abandoned copending application, by Frederick A. Glaski and James R. Humphrey, two of the applicants hereof, Ser. No. 645,278, filed June 12, 1967, for Method and Apparatus for Vapor Deposition, and illustrating simultaneously codeposited tungsten and rhenium; and

FIG. 6 is a schematic view of a switching system for automatically controlling the deposition cycle of the apparatus of FIG. 5.

DETAILED DESCRIPTION As previously noted, it is immaterial how the layered structure is formed on the mold or substrate. However, it is advantageous to employ the physical methods disclosed in assignees now abandoned copending application by Robert A. l-lolzl, Frederick A. Glaski and James R. Humphrey (the applicants hereof), Ser. No. 620,164, filed March 2, 1967, for Vapor Deposition Method and Apparatus," and in assignees now abandoned aforementioned application, Ser. No. 645,278.

In the method of the former application, the layers are obtained in extremely thin form by rapidly filling the CVD reaction chamber with the reactants at such high velocity, while pocketing the reactants therein, as to cause each layer to be substantially uniformly deposited over the entire surface of the hot mold or substrate. In the latter application and as shown in FIG. 5 hereof, the method is adapted particularly for relatively long objects and is effected by forming the layers in such manner that they are tapered, and the flow of the reactants is alternately reversed through the reaction chamber to cause the layers to taper in opposite directions and thus compensate for the taper and obtain an overall coating or plating of uniform thickness.

With respect to depositing layers of different pure metals, such as rhenium and tungsten, the conventional unidirectional flow method may be employed wherein, for example, an extremely thin layer of either pure tungsten or rhenium is first deposited on the hot surface by continuous flow of the reactants into one end (upstream) of the reaction chamber and out the downstream end, and then a gas of the other metal is introduced through the upstream end of the reaction chamber to deposit a layer of such metal; and this sequence is continued until a layered structure of desired thickness is obtained. However, except for relatively short objects, this method is not preferred because the layers will taper all in one direction and hence the overall layered structure will not be of uniform thickness, and subsequent machining is required This is due to the fact that the metal to be deposited thins out on the substrate from the upstream end of the introduction of the gaseous reactants toward the downstream end. i

As previously noted, the temperature which is maintained on the surface to be coated is immaterial as long as it is sufficiently high to effect deposit of the metals onto such surface by reduction of a gas of the metals by means of a reducing agent gas, such as hydrogen. Also, the temperature should be such that deposition of the respective metals is effected efficiently at the same constant temperature. In other words, conventional vapor deposition technique is employed.

In the case of deposition of pure tungsten and pure rhenium separately, a practical operating temperature range is about 900 to l,100 C., and desirably about l,000 C. However, for simultaneous codeposit of tungsten and rhenium a practical operating temperature range is about 600 C. to 700 C. and desirably about 650 C. The higher temperature for separately deposited layers of tungsten and rhenium, is governed by the fact that rhenium is more efficiently deposited at a much higher temperature but when the rhenium and tungsten are codeposited, the presence of the tungsten enhances deposit of the rhenium at a lower temperature. However, the deposition temperatures are not critical as long as efficient deposit of the respective metals is obtained.

It is general to effect the deposit of the respective metals from reactants comprising a gas of the metal and hydrogen gas; the gaseous metal reactant being usually in the hexafluoride state, such as tungsten hexafluoride or rhenium hexailuoride but other halogen gases of these metals may be employed.

To control interstitial fluorine content, it is advantageous in the invention hereof, but not necessary, to introduce with the reactant gases an oxygen source as disclosed and claimed in assignees copending application, Ser. No. 717,798, by Robert A. Holzl, an applicant hereof, filed Apr. 1, 1968, entitled Chemical Vapor Deposition Method and Apparatus, and Product; now (1.5. Pat. No. 3,565,676, granted Feb. 23, 1971 and one of the examples hereof discloses the introduction of a very small amount of oxygen. However, the invention is not dependent on oxygen introduction with the reactant gases.

As previously discussed, each layer should not be so thick that homogeneous dispersion of the respective metals among each other does not occur; and for this reason each individual layer should be below about 25 microns in thickness. The thinner the layers, the more effective is the diffusion. The minimum thickness is only governed by practical considerations for obtaining uniform plating. A practical minimum is about 0.5 micron.

In the formation of the layered coating structure, it is desirable in the case of tungsten and rhenium alloys to which this instant invention is particularly applicable, to have the: rhenium content in the ultimate alloy such that it varies from about 14 to 30 percent by weight rhenium, and desirably about 22 to 26 percent weight rhenium. This is so because if the rhenium content is too high, the resultant alloy may become brittle. with respect to the minimum amount of rhenium, namely, 14 percent by weight, if the rhenium content is much lower, then ductility of the resultant alloy will be impaired. To obtain this desirable range of rhenium with respect to tungsten, the quantities of the respective gases of these metals are adjusted in accordance with conventional CVD control technique.

Chemical vapor deposition results in formation of a columnar structure inherently present in all CVD deposits of metal, namely, columns of the metal which are substantially perpendicular to the mold or substrate surface upon which the metal is deposited. The heat treatment hereof results in effecting transformation of the columnar structure to an equiaxed grain structure as a result of diffusion of the respective metals in extremely thin layers thereof. This can be observed by comparison of FIG. 1 and 2, and FIGS. 3 and 4, respectively, wherein FIGS. 1 and 3 illustrate the deposit before heat treatment and FIGS. 2 and illustrate the transformation after heat treatment.

The heat treatment is effected on the layered deposit after it is removed from the reaction chamber, and separated from the mold or substrate in a conventional manner such as by pulling it off the mold in the case of a steel mold or by dissolving the mold in a strong acid, such as aqua regia.

With respect to the character of the heat treatment, it is only necessary to heat for a sufiicient time and at a sufficient temperature to cause the diffusion or, in other words, recrystallization and homogenization of the respective metal elements. After such homogenization, it is immaterial how high the temperature is raised or the length of time heat is applied, as long as the alloy is not heated to the extent where incipient fusion or melting occurs.

For example, with alloys of tungsten and rhenium in the range noted above, namely, about 14 to 30 percent by weight rhenium, recrystaJlization to the equiaxed state has been effected for 10 minutes at l,800 C. and for 1 hour at 2,000 O, and no difference in effect occurred. It is, however, desirable not to exceed a heat treatment temperature of about 2,500 C. as a precautionary measure. A suitable time and temperature for a tungsten-rhenium alloy in the range noted is about 60 to 10 minutes at a temperature of about l,800 C. to 2,200" C. It is not desirable to heat below the minimum indicated as then the recrystallization tothe very strong equiaxed state may not occur.

After the heat treatment, the recrystallized rhenium-tungsten alloy should be allowed to cool rapidly to ambient temperature because otherwise brittleness of the alloy might occur due to a further grain transformation as a result of cooling. The higher the rhenium content, the more likely the alloy may become brittle if not allowed to cool rapidly.

So that this rapid cooling may be effected, the alloy is advantageously heat treated after removal of the mandrel, by electrical resistance heating in an atmosphere inert to the alloy to prevent oxidation. Any suitable inert atmosphere can be employed; the preferred inert atmosphere being a mixture of about percent nitrogen and 15 percent hydrogen by volume, known as forming" gas. In other words, the alloy is used as a resistance element between two electrodes, and electric current is passed therethrough to heat it to the desired temperature. With such type of heating, heat will be dissipated quickly from the alloy as soon as the passage of current therethrough is terminated, and it will cool rapidly in a matter of 3 or 4 seconds to below a temperature of L200 C. At about this temperature, there is no danger of brittleness developing as the alloy cools down further to ambient temperature. Below about 22 percent rhenium content in the alloy, the cooling rate does not make so much difference in avoiding development of brittleness but above about 22 percent rhenium content, the rapid cooling is important in avoiding development of brittleness upon cooling.

The heat treatment could be effected in an inert atmosphere within a furnace if after such treatment the alloy is removed promptly and allowed to cool quickly to ambient temperature. However, there is a mechanical problem in removing the alloy from the furnace because of the high temperature thereof; and if the'alloy is allowed to remain in the furnace after the heat treatment has been completed, the furnace will not cool rapidly when application of heat thereto is terminated because of the high-thermal lag of such furnace.

Alloys of refractory metals, particularly rhenium and tungsten, are commonly used for the manufacture of shaped crucibles in which chemical reactions may be conducted or as tubes for conveying corrosive liquids. Also, such alloys find applicability, for example, in high-temperature environments, such as thermocouple probes or sheaths.

FIG. 5 illustrates a form of apparatus of the type disclosed in the now abandoned aforementioned copending application, Ser. No. 645,278, in which the method of this invention can be conducted to form an exterior coating on a tubular conventional steel mandrel or mold 2 which provides the substrate for the coating. The apparatus is more or less conventional, except for a valving arrangement to be'subsequently described to enable reverse flow of the reactants. In this connection, the reactants employed are those for providing a tungsten-rheniurn coating on the mandrel 2, but any other type of thermochemically deposited coating can be formed on the mandrel.

The apparatus comprises conventional glass tubing 11 usually of quartz or Vycor" glass clamped in a conventional manner to a suitable support (not shown), and which forms reaction chamber 12 through which How of the reactants is effected. Conventional rubber stoppers 13 are removably fastened in the respective ends of the chamber 12.

Mandrel 2 is resistance heated in the chamber by the passage of electric current directly therethrough from solid copper conducting rods 14 passing through the respective stoppers 13, and which are connected to copper cables 16 by means of clamps 17. Mandrel 2 is clamped between the ends of rods 14; and to allow for expansion of the parts under heat, the lower rod 14, shown in the FIG., is placed under tension by means of coil spring 18, to thus insure that it can move slightly through the associated stopper as expansion occurs.

For controlling flow of reactant gases into the chamber 12, an automatically operated, electromagnetically controlled inlet valve 21 is connected to one end of the chamber, and another similar inlet valve 22 is connected to the opposite end. These valves are also connected in tubing 23 for the conduction of the reactant gases and which has extensions 24 extending through the respective stoppers 13. Also connected in tubing 23 is an enlarged chamber 26 which serves as a mixing chamber for the reactants.

Mixing chamber 26 is connected to manifold inlet tubing 27 which is, in turn, connected to a source of reactants comprising source 28 of tungsten hexafluoride connected to a conventional flow meter 29, with a manually operable control valve 31 in manifold 27 after flow meter 29. Similarly, a source 32 of rhenium hexafluoride is connected to conventional flow meter 33 and another manually operable control valve 34; and a source of hydrogen 36, flow meter 37, and manually operable valve 38 are also similarly connected to manifold 27. Usually when the apparatus is started, it is desirable to purge the same with an inert gas and for this purpose, a source of argon 39 is connected to flow meter 41, and to manually operable valve 42 in manifold 27.

Means is provided for alternately periodically effecting flow of the reactants through the respective ends of reaction chamber 12 comprising tubing 51 connected to a constant source of vacuum provided by vacuum pump 52. Tubing 51 has extensions 53 extending through and gripped by the respective stoppers 13. The exhaust of reactants from one end of chamber 12 is automatically controlled by electromagnetically operated exhaust valve 54; and similarly, exhaust from the opposite end of chamber 12 is controlled automatically by electromagnetically operated exhaust valve 56. From the preceding it is seen that periodic reversal of flow of reactants through the reaction chamber 12 can be effected by operating the inlet and exhaust valves in such manner that inlet valve 21 and exhaust valve 56 are open when inlet valve 22 and exhaust valve 54 are closed, and vice versa.

Means is provided for automatically operating the respectivesets 21, 56 and 22, 54 of electromagnetically controlled valves in proper sequence. Referring to FIG. 6, a conventional, electrically operated motor-timer 61 is provided connected to drive shaft 62 which has cam discs 63, 64, 66 and 67 fixed for rotation therewith; the respective cam discs having cams 68, 69, 71 and 70 which, respectively, open and close spring-pressed switch arms 73, 74, 76 and 77 connected by wiring 78, 79, 81 and 82 to open and close the sets of valves in the sequence pointed out above.

EXAMPLE 1.

The following is a typical example of an embodiment of the invention for the production of tungsten-rhenium heat-resistant tubing desirable for use for the conduction of fluids and gases under high-temperature environments of over 2,000" C.

Reaction chamber 12 is a cylinder 6 feet long with an inside diameter of about l 9% inches. Mandrel 2 is of stainlesssteel, 60 inches long by 0.030 inch external diameter. in accordance with conventional practice, reaction chamber 12 is first purged free of oxygen by flushing a number of times with argon. This is readily done by closing valves 31, 34 and 38 with valve 42 open, and alternately operating the sets of valves 21, 56 and 22, 54. in this connection, the vacuum source 52 is about 28 to 29 inches of mercury (vacuum).

After such purging, which is effected in about 15 minutes, mold 2 is heated to a deposition temperature and it is maintained at that temperature throughout the course of the deposition cycle.

Argon control valve 42 is closed; and the hydrogen control valve 38 is opened while the rhenium hexafluoride and tungsten fluoride control valves 34 and 31, respectively, remained closed. A hydrogen flow of l,200 standard (standard temperature and pressure conditionsSTP) cc.lminute, as monitored by flow meter 37, is then fed into chamber 12 by sequential operation of the sets of valves 21, $6 and 22, 54. Thy hydrogen is allowed to flow alone to insure a uniform charge of hydrogen in chamber 12, which is in about 2 minutes. Then the tungsten hexafluoride control valve 31 is opened to permit a flow rate of tungsten hexafluoride of about 300 standard cc./minute, and the rhenium hexafluoride control valve 34 is opened in about one-half minute after opening of the tungsten fluoride control valve to pennit a flow rate of about 90 standard cc./minute, the proportions being such that each layer has about an 18 percent by weight rhenium content (plus or minus 1% percent) with the rhenium content richer adjacent the inner surface of each layer because the rhenium inherently deposits on the mandrel faster than the tungsten.

The timing of the automatic opening and closing of the respective sets of flow control valves 21, 56 and 22, 54, is suchv that the mixture of gases flows in each direction for about 15 seconds; and the total time of deposition is about I l minutes. This results in a coating on the mandrel of about 44 layers.

The deposit of tungsten and rhenium on 'mandrel 2 is about 55 inches long, with'an overall uniform thickness of the 44 layers of about 0.007 inch (7-mil about 175 microns) plus or minus 0.0005 inch (0.5 mil). Thus, each layer has an average thickness of about 4 microns l75 microns divided by 44).

After shutting down the apparatus, a free standing tube is finally produced by mechanically pulling one end of the stainless steel mandrel 2 from the deposited coating. In this connection, mandrel 2 is desirably a hollow tube so that in case it sticks to the coating when attempt is made to pull it out, acid such as hydrochloric acid or aqua regia, can be introduced within the tube to etch it out.

After separation ofthe layered rhenium-tungsten tubing from the mandrel, it is clamped between its ends by electrical conductors in an enclosed glass tube as in the deposition process, which contains an inert atmosphere of percent nitrogen and 15 percent hydrogen by volume, and electrical current is passed through the tubing to bring it to a temperature of about l,800 C., and such temperature is held for about it) minutes. Upon shutting off the current, the tubing cools quickly in about 3 seconds to about L200" C., and to about ambient temperature in about 2 minutes thereafter. The transformation of the columnar grain structure to a strong equiaxed grain structure occurs, resulting in a homogeneous nonbrittle alloy exhibiting all the characteristics of a wrought" alloy having marked room-temperature ductility.

EXAMPLE 2 The following is a typical example wherein tubing is made by the alternately flow system of FIGS. 5 and 6 but wherein an alloy of tungsten and rhenium is formed by flow of tungsten hexafluoride alone in one direction, and then flow of rhenium hexafluoride alone is effected in the same direction. Then the direction of both the tungsten hexafluoride alone and the rhenium hexafluoride alone are reversed, and the reversals continued alternately to build up a coating or plating of the desired thickness with alternate layers (every other layer) of pure rhenium and pure tungsten.

The apparatus is the same as in FIGS. 5 and 6 except that the mixing chamber 26 is omitted and additional automatically controlled valves 86 and 87 (shown in phantom lines in FIG. 5) are employed after valves 31 and 34, respectively, and which are sequentially operated to provide for separate flow of the respective hexailuorides. Also, a minute controlled amount of water-saturated air is continuously introduced into the system during the deposition cycle to control interstitial fluorine content, as disclosed in the aforementioned application, Ser. No. 717,798 now U.S. Pat. No. 3,565,676, granted Feb. 23, l97l.

As with respect to example i, reaction chamber 2 is first flushed with argon. The following are the run conditions for the instant example:

Reaction chamber-diameter Reaction chamberlength Mandrel material 1% in. inside diameter stainless steel0.083 in. outside diameter and 24 in. long 28.5 in. mercury (absolute) Same procedure. as in example l at 380 ccJminute (Standard) 250 cc. per minute (standard) M andrel temperature Vacuum Argon flushing 200 cc. per minute (standard) seconds in each direction 5 seconds in each direction 65 minutes Tungsten hexafluoride and then rhenium hexafluoride were run successively in one direction, and then run successively in reverse direction.

The number of individual layers of rhenium and tungsten which are formed is approximately 312, and the overall thickness of the resultant rhenium-tungsten layered coating is about 0.036 inch (960 microns). Each rhenium layer averages about l.5 microns thick (101 micron) and each tungsten layer about 4.0 microns ($0.3 micron). The mandrel being of stainless steel is removed by pulling it out of the coating, and

heat treatment of the coating is effected by electrical resistance heating in the same manner as in example 1 at 2,000

lts grain structure is transformed to a strong equiaxed rain, as can be seen by comparison of FIGS. 3 and 4 w lCh are photomicrographs of the product of this example before and after heat treatment It has marked room temperature ductility in that it can be plastically deformed after the heat treatment as much as to about a l$ angle without breaking. The resultant tubing is suitable as a hightemperature resistant thermocouple probe sheath. Also, such alloy is suitable for the uses previously discussed.

The above examples are merely exemplary because as previously pointed out the invention can be employed with any physical method of effecting CVD, as long as the layered structure is formed wherein the adjacent layers are of predominantly different metals and the layers are so thin as to result in diffusion of the metals through substantially all of the adjacent layers upon heat treatment.

We claim:

1. In a chemical vapor deposition process wherein an alloy consisting essentially of tungsten and rhenium metals having a rhenium content of about 14 to 30 percent by weight is formed from reactants including said metals which are coated over a surface of an object by application of heat to said object and a columnar grain structure is inherently formed on said object, the improvement which comprises depositing said tungsten and rhenium metals over said surface in layers wherein adjacent layers are predominantly a different one of said metals, and each of such adjacent layers is below about 25 microns in thickness to cause diffusion and homogenization of said tungsten and rhenium through substantially all of said adjacent layers upon subsequent heat treatment, and after formation of such layered coating heattreating the same at a temperature of about 1,800 C. to 2,500 C. and for a length of time sufficient to cause said diffusion of said tungsten and rhenium and convert said columnar grain structure to an equiaxed grain structure. 1

2. In a chemical vapor deposition process wherein an alloy consisting essentially of tungsten and rhenium metals having a rhenium content of about 14 to 30' percent by weight is formed from reactants including said metals which are coated over a mold surface by applications of heat to said mold surface which shapes said alloy and a columnar grain structure is inherently formed on said mold surface, the improvement which comprises depositing said tungsten and rhenium metals over said mold surface in layers wherein adjacent layers are predominantly a different one of said metals, and each of said adjacent layers is below about 25 microns in thickness to cause diffusion and homogenization of said tungsten and rhenium through substantially all of said adjacent layers upon subsequent heat treatment, and after formation of such layered coating heattreating the same at a temperature of about 1,800 to 2,500C. and for a length of time sufficient to cause said diffusion of said tungsten and rhenium and convert said columnar grain structure to an equiaxed grain structure.

3. The method of claim 1 wherein the heat treatment is electrical resistance heating.

4. The method of claim 1 wherein the rhenium content is about 22 to 26 percent by weight.

5. The method of claim I wherein the layers are alternately of tungsten and rhenium separately deposited on said surface.

6. The method of claim 1 wherein each of the layers is codeposited tungsten and rhenium rich in one of such metals. 

2. In a chemical vapor deposition process wherein an alloy consisting essentially of tungsten and rhenium metals having a rhenium content of about 14 to 30 percent by weight is formed from reactants including said metals which are coated over a mold surface by applications of heat to said mold surface which shapes said alloy and a columnar grain structure is inherently formed on said mold surface, the improvement which comprises depositing said tungsten and rhenium metals over said mold surface in layers wherein adjacent layers are predominantly a different one of said metals, and each of said adjacent layers is below about 25 microns in thickness to cause diffusion and homogenization of said tungsten and rhenium through substantially all of said adjacent layers upon subsequent heat treatment, and after formation of such layered coating heattreating the same at a temperature of about 1,800* to 2,500* C. and for a length of time sufficient to cause said diffusion of said tungsten and rhenium and convert said columnar grain structure to an equiaxed grain structure.
 3. The method of claim 1 wherein the heat treatment is electrical resistance heating.
 4. The method of claim 1 wherein the rhenium content is about 22 to 26 percent by weight.
 5. The method of claim 1 wherein the layers are alternately of tungsten and rhenium separately deposited on said surface.
 6. The method of claim 1 wherein each of the layers is codeposited tungsten and rhenium rich in one of such metals. 